Transfer cases containing the remains of Army Spc. Tyler J. Orgaard and Army Sgt. 1st Class Darren M. Linde sit on a loader during a prayer at Dover Air Force Base, Del. According to the Department of Defense, Orgaard, 20, of Bismark, N.D., and Linde, 41, of Sidney, Mont., both died Dec. 3, 2012, in Lashkar Gah City, Helmand province, Afghanistan of wounds sustained when enemy forces attacked their unit with an improvised explosive device. The shrinking US combat role in Afghanistan has had the unwanted effect of giving insurgents more time and space to plan deadlier attacks using bigger improvised bombs. So while the number of attacks has declined, attacks are more lethal, Pentagon officials say. (ASSOCIATED PRESS / Steve Ruark, File)

WASHINGTON (AP) — The shrinking U.S. combat role in Afghanistan has given insurgents an opening to devise and carry out deadlier attacks using bigger improvised bombs against U.S. and coalition military vehicles and bases, American officials say.

With fewer U.S. forces patrolling road networks beyond their bases — and with the grounding of eye-in-the-sky surveillance balloons known as aerostats — Taliban fighters are adapting their tactics, according to officials at a Pentagon agency that tracks attacks that use improvised explosive devices, or IEDs.

“Insurgents are able to invest more time in preparing and staging an attack, and when we see an effective attack, it tends to be more lethal to our forces,” said Al Sweetser, chief of the operations analysis division at the Joint IED Defeat Organization, which has its own team of analysts on the ground in Afghanistan.

So while the number of IED attacks against U.S. and coalition troops — as well as the total casualties they cause — has declined, certain attacks can be more elaborately planned, precisely targeted and more lethal because the insurgents have time and room to prepare, Sweetser and other American officials said.

The Pentagon has invested billions of dollars to develop gadgets, such as hand-held ground-penetrating radar systems, and techniques to find IEDs before they explode and to mitigate damage from those that can’t be stopped. The evolving struggle began a decade ago in Iraq with insurgent groups countering high-tech U.S. weapons with relatively cheap and surprisingly effective methods of killing and maiming. A hallmark of the insurgents’ use of IEDs in Afghanistan has been their ability to adapt to changing circumstances.

A recent IED strike near the southern provincial capital of Kandahar killed five American soldiers in an armored vehicle in one of the more deadly examples of how the Taliban have taken advantage of a changing military landscape.

Insurgents took the time and effort to tunnel underground and place an unusually large improvised explosive well below the surface, said David Small, spokesman for the counter-IED agency. They also had time to plan where to position themselves to remotely detonate the bomb, Small said.

Hel said other details about the attack are classified, but it is believed the insurgents had learned through observation that reduced coalition patrolling gave them more time to plan this attack.

Among other recent incidents:

— An IED attack on a coalition base in Zabul province in April killed three soldiers from the 1st Armored Brigade, 3rd Infantry Division, and wounded three others. Another attack in April killed three British soldiers.

— Five soldiers from the 1st Brigade, 1st Armored Division were killed when an IED hit their Stryker vehicle in Kandahar province in May. Three other members of the same brigade were among four soldiers killed in a separate attack later that month, also in Kandahar.

— In June a suicide bomber driving a truck loaded with explosives attacked an isolated NATO base in Helmand province, killing seven Georgian soldiers and wounding nine. That was the deadliest single attack of 2013.

“These are precision attacks employing suicide, vehicle-borne and command-wire IEDs, all of which require longer planning and preparation than the more common IEDs encountered,” Small said.

Afghan forces are taking the brunt of insurgent violence this year as U.S. troops pull back from a combat role to a support mission in line with plans to end the American and coalition combat role by December 2014. Yet IED attacks are taking a relatively larger toll on the smaller force that is still operating there. Deaths from IEDs alone from April through June represented 9 percent of all coalition casualties, including those wounded in action, Sweetser said. That compared to 7 percent in the same period in 2012.

Because U.S. and coalition troops are not as active beyond their bases as they had been previously, prior to the Afghans taking the lead combat role across the country, Taliban fighters “are coming to us and attacking us on our bases,” Sweetser said.

A spokesman for coalition forces in Kabul, Lt. Col. Will Griffin, acknowledged the increased lethality of some attacks but said, “We don’t necessarily identify that as a trend.” He said a “full spectrum” of tools designed to counter the threat of IEDs is still available to coalition troops even as they shift to a back-seat role.

U.S. officials also are now grappling with a time lag in learning details about IED attacks, since the Afghans are in the combat lead and use more rudimentary means of reporting up their chain of command, Sweetser said. He said it now can take many weeks, rather than several hours, to learn details of IED attacks.

There are now about 60,000 U.S. troops in Afghanistan, down from 87,000 a year ago. The number is scheduled to shrink to 34,000 by February. President Barack Obama has yet to announce whether he intends to keep some number of American forces in Afghanistan after 2014 to train Afghans and to hunt extremists.